A new box opens and you'd THINK you need to click on "select files", but NO, you need to scroll below the box and click on "Add Files".

A new window opens and you can navigate to the directory on your computer where the file or picture is stored.

Or, in Windows you can open Explorer and drag the files to the box. 

Then you click on UPLOAD and they will be inserted in your post.

All that scrolling is a little cumbersome, but it works.

Here's a little video:

And here's a screenshot of the upload screen:

Please let me know if you have any questions!